Scientists have long been concerned that global warming may push Earth's climate system across a "tipping point," where rapid melting of ice and further warming may become irreversible -- a hotly debated scenario with an unclear picture of what this point of no return may look like.

The National Research Council says that even gradual climate change can have abrupt impacts in human infrastructure and ecosystems if critical thresholds are crossed, so an early warning system needs to be developed.

There have been instances in Earth history when average temperatures have changed rapidly, as much as 10 degrees Celsius (18 degrees Fahrenheit) over a few decades, and some have speculated the same could happen again as the atmosphere becomes overloaded with carbon dioxide.

A pivoted catch designed to fall into a notch on a ratchet wheel so as to allow movement in only one direction (e.g. on a windlass or in a clock mechanism), or alternatively to move the wheel in one direction.